Optical fiber structures have traditionally been created using glass or plastic materials such that a cylindrical waveguide core of a particular material with a specific refractive index is surrounded by a cladding material of lower refractive index than the waveguide core. When optical energy propagating in the high refractive index waveguide core has an appropriate combination of wavelength and propagation angle, the energy can be totally reflected at the interface between the high refractive index waveguide core and low refractive index waveguide cladding. Such reflection within a high refractive index waveguide core is normally referred to as total internal reflection (TIR).
Traditionally, the materials utilized for the construction of optical fibers have been bulk materials, whose optical and electrical properties were intrinsic properties of the constituent elements. Manufacturing techniques for drawing fiber from preform structures has previously been limited to producing fibers having either smooth gradient refractive index distributions or simple step-wise radial refractive index distributions that were relatively constant in an azimuthal direction. Recent advances in optical fiber fabrication have allowed for the construction of more elaborate spatial refractive index distributions on the micro and nano scale in both radial and azimuthal directions with respect to the fiber axis.
Photonic crystal fibers are a class of micro-structured optical fiber in which an engineered periodic spatial distribution of materials provides for optical properties that are not present, or in some cases not possible, in the constituent bulk materials alone. Photonic crystal fibers have been designed to operate either through traditional index guiding methods, where a solid guiding core has a higher refractive index than the photonic crystal cladding, or on the principle of a photonic bandgap structure that confines optical propagation in a guided mode within the core through phase matched reflections of energy from multiple layers of material. Photonic crystal fibers can include these periodic variations in one dimension, such as in the concentric radial rings of a Bragg fiber, or in two dimensions, wherein a regular array of high refractive index or low refractive index longitudinal cylindrical (rod-like) structures is distributed through a bulk background material across the cross-sectional area of the fiber. These periodic index variations and geometric features have sizes on the same order of magnitude as the wavelength of optical energy being guided. In some cases, the low refractive index cylindrical structures may be constituted by holes formed from voids in the bulk background material. Based on the nature of a photonic bandgap that is created by a regular periodic array of high or low index structures, inclusion of a defect within the array of structures can allow for optical energy to be confined within such a defect. In some cases, the use of photonic bandgap effects can allow for light to be confined within a low refractive index core, optionally an air core, surrounded by material whose effective refractive index is of a higher value. While hollow core photonic crystal fibers avoid various optical nonlinearities that can occur at high power in solid core fibers and provide a method of fast optical communication relative to traditional solid core fibers, they generally produce optical propagation speeds that are much lower than free space optical communication due to the influence of the cladding's photonic bandgap structure on the propagating mode. Consequently, there exists a need for a hollow core optical waveguide that provides the low latency benefit associated with free space optical communications within an optical fiber structure.